Seat belt devices are indispensable devices for seats used for rapidly moving vehicles, or the like, in order to restrain occupants (wearing seat belts) from rapid movement due to sudden acceleration that occurs in the event of vehicle collision. Such seat belt devices consist of a seat belt, a seat belt retractor and a buckle as main components, wherein the seat belt retractor is a principal mechanism component.
The seat belt retractor has a winding device called a spool and is adapted to contain the seat belt as it pulls and winds the seat belt onto the spool by means of a spring force. In order for an occupant to wear the seat belt, the occupant withdraws the seat belt from the seat belt retractor by pulling a tongue plate attached to the seat belt and engaging the tongue plate with a buckle mounted on one side of the seat, thus giving the state where the occupant M is restrained by the seat belt.
In recent years, these seat belt retractors have generally been provided with pretensioners for further increasing the restraint imparted to occupants. When restraining operation of the seat belt devices is required, the pretensioners remove slack of the worn seat belts to give a proper restraining state while imparting tension to the seat belts so as to maintain the restraining state, thus increasing the restraint imparted to the occupants.
Some types of mechanism are known for removing slack and generating tension in a pretensioner. A typical example of such a mechanism is disclosed in Japanese patent application publication No. 2005-306111 (“hereinafter, called the “ball-type application”).
The ball-type pretensioner removes slack of the seat belt by rotating a spool by means of the ball train being moved by gas discharged from a gas generating device, while giving a predetermined tensile state of the seat belt by means of the gas pressure on the ball train imparting resistance on the spool against belt withdrawal for maintaining a restraining state where slack is removed.
More specifically, the ball-type pretensioner includes a ball guide pipe that contains a plurality of balls so as to form a ball train provided with a piston component at its proximal end, and a gas generating device is connected to the proximal portion of the ball guide pipe. The gas generating device ignites to discharge high-pressure gas when an acceleration sensor detects acceleration equal to or larger than a predetermined value. Subsequently, the gas pressure is transmitted through the piston component to the ball train, causing the ball train to move inside the ball guide pipe. This movement of the ball train is then transmitted to the spool to drive the spool to rotate while imparting predetermined resistance on the spool against belt withdrawal by the gas pressure exerted on the ball train.
The mechanism for transmitting, to the spool, movement of the ball train and gas pressure on the ball train is, for example, formed of a combination of ring gear and pinion gear. Specifically, the ring gear includes external teeth that are formed on the outer periphery of the ring gear for transmitting movement of the ball train and gas pressure on the ball train through engagement of the moving ball train, and internal teeth that are formed on the inner periphery of the ring gear for engaging with the pinion gear. As the ball train starts moving, the ring gear engages with the pinion gear and rotates, with the result that the pinion gear rotates. The rotation of pinion gear is transmitted to the spool so that the spool rotates in the direction in which the spool winds the seat belt. The amount of rotation by which the spool winds depends on the degree of slack of the seat belt at that point in time. When the belt slack has been removed so as to give a proper restraining state, the spool stops its rotation and maintains a state where predetermined resistance is imparted against belt withdrawal by gas pressure on the ball train.
The above described ball-type pretensioner is advantageous in that it effectively performs functions desired for pretensioners. However, there still remains room for further improvement in its function. Specifically, the above-described ball-type pretensioner has a problem associated with removal of slack of the seat belt.
The amount of rotation by which the spool winds when the pretensioner removes belt slack, as described above, depends on the degree of slack of the seat belt at that point in time. When the degree of slack exceeds a normal range, all the balls of the ball train inside the ball guide pipe may be used. As a result, there is a possibility that transmission of gas pressure through the balls to the spool is disabled, that is, the application of tension to the seat belt for restraint is disabled. Slack of the seat belt is related to the type of clothes that an occupant is wearing. In other words, if the occupant is wearing thick clothes, as a result, larger belt slack must be removed due to the clothes. In such a case, excessive belt slack tends to occur.
Accordingly, there is a need for a pretensioner capable of applying tension to a to seat belt to restrain an occupant when the seat belt is exhibiting excessive slack.